Chemical vapor deposition (CVD) systems are used to form a thin, uniform layer or film on a substrate such as a semiconductor wafer. During CVD processing, the substrate is exposed to one or more chemical vapors such as silane, phosphane, diborane and the like, and gaseous substances such as oxygen. The gases mix and interact with the other gases and/or the surface of the substrate to produce the desired film. The desired reactions generally occur at elevated temperatures, for example 300.degree.-500.degree., with the substrate and chamber being heated to the appropriate temperature for a selected process. In many applications including semiconductor processing, film characteristics such as purity and thickness uniformity must meet high quality standards. The substrate is positioned in a clean, isolated reaction chamber to obtain high quality films. The CVD systems typically employ injectors which deliver the gaseous substances directly to the surface of the substrate. An exhaust system removes waste products such as unreacted gases and powders formed during the reaction from the reaction chamber. The reaction chamber must be periodically cleaned to remove films which are deposited on the exposed surfaces of the chamber over time, eliminating sources of particulate contamination which may become embedded in the film.
The injection ports are typically positioned less than one inch from the surface of the substrate, for example 1/8 inch to 1/2 inch. With this limited clearance between the injector and the substrate surface, the surfaces of the injector will become coated with the material produced during the reactions. To reduce the amount of build-up in this area, some CVD systems include shields which are positioned in front of the injectors and exhaust port. An inert gas such as nitrogen is delivered through a perforated screen to slow the rate at which materials accumulate on shield. The high temperatures in the reaction chamber create thermal stresses in the perforated screen which may cause the screen to buckle after a period of time. The buckling disrupts the uniform flow of nitrogen through the screen, leaving portions of the screen unprotected against the accumulation of deposition materials. The ability of the screen to deliver nitrogen to the reaction chamber is further reduced as the screen becomes coated with deposition materials, requiring removal and cleaning or replacement of the shield. Thus, the collapsed walls shorten the useful life of the shield. A protective gas shield for CVD systems which will withstand the high temperatures within the reaction chamber is desirable.